Protection devices of the above kind, which include thermistors of a ceramic type, are previously known from patent publication GB-A-1570138. Ceramic thermistors may be BaTiO.sub.3 -based or V.sub.2 O.sub.3 -based. Such thermistors are described in, for example, U.S. Pat. No. 4,642,136 and in the article "A New PTC Resistor for Power Applications", by R S Perkins et al, published in the journal IEEE Transactions on Components, Hybrides and Manufacturing Technology, Vol. CHMT-5, No. 2, June 1982, pp. 225-230. A disadvantage with ceramic thermistors is that they exhibit negative temperature coefficient when the temperature exceeds a certain value. In addition, a ceramic thermistor has a tendency to crack when traversed by a short-circuit current because of the thermal and mechanical stresses to which it is subjected when it changes to its high-resistance state. Also, it is relatively expensive.
Another type of thermistors intended for overcurrent protection are known from, inter alia, patent publications EP-A-0038716 and EP-A-0087884. Thermistors of this type are built up from a polymer material, for example high-pressure polyethylene, containing particles of an electrically conductive material, for example carbon black. Alternatively, the electrically conductive filler may consist of V.sub.2 O.sub.3 material dispersed as described in the article "V.sub.2 O.sub.3 Composite Thermistors" by D Moffatt et al, published in Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics, 1986, pp. 673-676. A polymer-based thermistor of the above-mentioned kind has several advantages compared with a ceramic one. For example, its resistance in the temperature range in question is monotonously increasing with the temperature and the resistance increase is considerably greater. In addition, it is considerably cheaper. However, commercially available thermistors of this type are designed for relatively low rated voltages, for example 50-100 V, and cannot therefore be readily used in distribution networks.
In EP-A-0363746 a device for overcurrent protection is described, which includes a polymer-based thermistor of the above kind. This device is designed so as to function both as a motor and a short-circuit protective means and it may be used in networks with an operating voltage of, for example, 230 V in which voltage peaks of the order of magnitude of 400-500 V may occur, for example in case of short-circuit breaking. The maximum presumptive short-circuit current which may be broken with this device is, however, limited, as is the possibility of adapting the tripping characteristic of the device to the equipment to be protected.